1. Field of the Invention
The present invention relates to a charge pump circuit.
2. Description of the Related Art
In a recent electronic apparatus such as a mobile phone or a PDA (Personal Digital Assistants), a device, which requires a drive voltage higher than a battery voltage, such as an LED (Light Emitting Diode) used for a liquid crystal backlight is mounted. In each of these compact information terminals, for example, a Li-ion battery is frequently used, the output voltage is about 3.5 V in general and about 4.2 V in a full charged state. For this reason, the LED requires a voltage higher than the battery voltage as the drive voltage. In this manner, when the voltage higher than the battery voltage is necessary, the battery voltage is boosted by using a charge pump circuit or a switching regulator, and a voltage required to drive the LED is obtained.
The charge pump circuit generates an output voltage obtained by multiplying an input voltage by a predetermined boost ratio. For example, when the battery voltage is 3 V and the boost ratio is twice, an output voltage is fixed to 6 V. Therefore, when a load circuit requires a drive voltage lower than 6 V, a power transistor is inserted on an input side or an output side of the charge pump circuit to configure a regulator. An ON-resistance of the regulator must be adjusted to adjust the output voltage. A related art is described in Japanese Patent Application (Laid Open) No. 2000-262043.
The present inventors considered a pulse modulation type charge pump circuit which modulates an ON-time of a switching device of a charge pump circuit to stabilize an output voltage and accordingly recognized the following problem.
As in a conventional art, when a charge pump circuit is combined to a regulator using a power transistor the ON-resistance of which is adjusted, a reference voltage of the regulator is moderately changed to make it possible to soft-start the charge pump circuit.
However, in control of an ON-time of a switching device of a charge pump circuit by pulse modulation without using a regulator, when the switching device is turned on, even though the ON-time is short, a large electric current (rush electric current) flows in a flying capacitor or an output capacitor having a small amount of electric charge. Such a problem may occur in not only a pulse-modulation type charge pump circuit but also in a fixed-duty type charge pump circuit.